1. Field of the Invention
The invention relates to a medical method and apparatus for measuring in non-invasive manner the concentration of blood constituents, particularly the hemoglobin concentration or oxygenation of the blood in large blood vessels.
2. Description of the Prior Art
Living tissue is largely transparent to electromagnetic radiation in the red and infrared range (wavelength 550 nm<λ<1000 nm). This so called “biological window” is limited towards longer wavelengths by strong absorption bands of water and towards shorter wavelengths by those of hemoglobin. It is in principle possible in this range to “see into” the tissue in depths ranging from a few mm to a few cm.
Over the last decade, so called pulsoximetry has evolved to become one of the most important monitoring methods for observing the patient in the intensive care unit and operating theater. The oxygenation of the blood is measured, i.e. the ratio of the concentration of the oxygen-containing hemoglobin to the total hemoglobin. This involves the measurement of the absorption in transmitted light or remission in backscattered light on tissue with a good blood flow (e.g. fingertip or ear lobe) at two different wavelengths. The wavelengths used are normally around λ=660 nm (where the oxygen-free hemoglobin is much more strongly absorbed than the oxygen-containing hemoglobin) and around λ=940 nm (where the relationships are reversed). During the measurement, use is made of the absorption signal modulation produced by the heart beat. The alternating signal is added to the arterial component of interest, and the absorption by venous blood and tissue to the equisignal present as background. The determination of the oxygenation as a relative quantity is possible with an adequate precision for clinical use, despite certain difficulties in practice.
However, medical literature proves the urgent need for a bedside, i.e. continuous, non-invasive determination of the oxygenation of hemoglobin in large blood vessels. Blood examination in large, so called “central”, i.e. heart-near vessels, is impossible with the hereto standard methods for determining oxygenation due to the so called “centralization”, (i.e. inadequate blood circulation of the periphery) of emergency patients. There is a need for a method which can be used arterially, e.g. on the internal carotid artery, and if possible, also on large veins, e.g. the internal jugular vein, because the difference in oxygenation provides important information on the oxygen supply of the brain.
The difficulty of a measurement on large blood vessels is that their central position in the body renders a transmission measurement impossible, whereas, remission mainly takes place by diffuse backscattering of photons. A distinction is made between ballistic or quasi-ballistic photons, which are subject to little or no interaction with the tissue, and therefore leave the latter first, and diffuse photons whose path through the tissue is characterized by numerous scattering processes. Ballistic photons are of minor importance for blood analysis. For human tissue, the scattering coefficient μs is much larger than the absorption coefficient μa. Thus, μs is typically approximately 10 cm−1 so that for layer thicknesses of ≧1-2 mm, no light focus can be produced in the scattering medium. The illumination of lower lying layers consequently takes place quasi-isotropically. The essential problem is the association of backscattered light with a specific location of the scattering.
DE 196 40 807 A1 proposes apparatuses for the detection of diffuse photons which measure in the backscattering direction, i.e. in the vicinity of the light sources, and are used for determining the oxygen concentration in the blood and tissue. Use is made of an empirically known link between the spacing of the exit point of backscattered photons from the entrance point (light source, fiber end) and the average penetration depth of said photons on their path through the tissue for controlling the observation depth through the choice of said spacing.
DE 196 34 152 A1 uses a very similar measuring setup, and the fact that components of the irradiated, coherent light are phase or frequency-shifted by elastic and inelastic scattering processes and are superimposed with the undisturbed components, leads to a speckle pattern. The spatially resolved measurement of the speckle pattern permits an analysis of the exiting stray light with respect to its power spectrum compared with that of the irradiated light. Thus, e.g. through repeated frequency shifts through inelastic scattering on blood, information can be obtained about the average number of scattering processes per photon on the light path through the tissue. The use of a filtering procedure then makes it possible to discriminate photons which have had a predetermined minimum of maximum number of scattering processes, and consequently, have a relatively strong localized penetration depth.
WO 02/08740 A2 describes the advanced prior art for a measuring apparatus with backscattered photons. On the basis of the known interaction of light with the ultrasonic field present in the tissue, conclusions are drawn from the resulting influencing of the phases of the electromagnetic waves with respect to the precise position of the responsible scattering centers in a three-dimensional measuring area. An ultrasonic field suitable for this purpose is produced either by a single movable or a complete array of sound sources in contact with the tissue. What is important for the necessary complex structure of the ultrasonic field is the precise control of the sources with respect to the maintaining of phase delays and repetition times, and/or frequency differences. The extensive data analysis on the detector side is equally complicated. The apparatus is provided for 3D picture-giving tomography in connection with the blood supply of tissue, and is vital for combating tumors.
It is common to all the aforementioned methods that the contribution to the measuring signal of blood from the interior of a large blood vessel cannot be considered in isolation or, in the case of WO 02/08740 A2, can only be established with considerable effort and expenditure. A simple, robust, fast and inexpensive system is necessary for the continuous monitoring of emergency patients. The problem of the invention is to make this available.